Apparatuses for producing thin cast strip include a twin drum-type continuous casting apparatus wherein molten metal is fed to a pouring basin formed by a pair of cooling drums and a pair of side weirs which are pressed to both sides of the cooling drums, for continuous casting into a thin cast strip. With this type of apparatus there is no need for a multi-step hot rolling process and the final product shape may be obtained with only light rolling, thus allowing a simpler rolling process and apparatus, and making possible a vast improvement in productivity, and in cost, compared to conventional production processes which involve hot rolling.
An example of a twin drum-type continuous casting apparatus is shown in FIG. 1. This apparatus has a pair of cooling drums 1, 1 placed parallel to each other at an appropriate spacing, with a pouring basin 3 formed by contacting side weirs 2, 2 (front one not shown) made of a refractory material, to both edges of the cooling drums. When molten metal M is fed to the pouring basin 3 through a pouring nozzle 4, the fed molten metal M contacts the cooling drums 1, 1 forming solidified shells 5, 5 around the cooling drums 1, 1. The solidified hells 5, 5 are integrated and pressed together at the position where the rotating cooling drums are closest to each other, i.e., the closest position of the cooling drums, to form a thin cast strip 6 with the prescribed thickness, and the thin cast strip 6 is fed out continuously below the cooling drums.
FIG. 2 shows an embodiment of the cooling drum described above. The cylinder section of the cooling drum 1 comprises a sleeve 10 and a base 11, and both sides of the cylinder section are connected to a rotating shaft 7. The sleeve 10 has a plurality of cooling water channels 12 across the entire perimeter face 15 of the cooling drum, and cooling water L is pressure-pumped from inlets 13 through the cooling water channels 12 and discharged from discharge outlets 14. The heat of the molten metal contacting with the perimeter face 15 of the cooling drum is absorbed by the cooling water L through the sleeve 10 and discharged out of the system.
For the material of the sleeve 10 there is usually selected a metal with good heat transfer, such as copper or a copper alloy, for more rapid heat removal from the molten metal. Also, as shown in FIG. 3, the outer perimeter face of the sleeve 10 usually has a plated layer 16 of nickel or cobalt, which has lower heat transfer than the sleeve 10 but good mechanical durability, formed as an outer protective layer in order to control the cooling rate of the thin cast strip.
One problem with continuous casting using the cooling drums described above is that a drum gap 9 formed by the closest position of the cooling drums becomes non-uniform along the widthwise direction of the cooling drum, due to heating of the cooling drum 1 by the molten metal which results in its thermal expansion and swelling into a barrel shape. When the solidified shells 5, 5 are pressed at the drum gap 9 formed by the closest position of the cooling drums in this non-uniform shape, the pressure force on the solidified shells 5, 5 becomes non-uniform, thus making the cast thin casting strip 6 non-uniform in the widthwise direction while also producing a non-uniform cooling rate of the thin casting strip across the width and generating defects such as cracks and wrinkles in the thin cast strip surface.
In order to overcome this problem concerning the shape of thin cast strips, there has been disclosed in Japanese Unexamined Patent Publication No. 61-37354 a method of offsetting the thermal expansion by adding to the cooling drum 1 a concave-shaped drum crown which is concave at the center. Hereunder this concave shape on the cooling drum will be referred to as the "drum crown", and the degree of the drum crown means the degree of the concavity formed at the outer perimeter face of the cooling drum and will be defined to mean the difference between the radius of curvature of the center portion in the width-direction and that of the most edge portions of the cooling drum.
The degree of the convex crown of the thin cast strip may be adjusted by adjusting the degree of the drum crown according to the method described in the above-mentioned publication, and, in fact, the adjustment of the degree of convex crown by other methods involves very a complicated drawing step after casting and an increased cost. For this reason, a drum crown must be added to the cooling drum 1 in the continuous casting apparatus employing the cooling drum.
Nevertheless, when cast strip is produced with a cooling drum provided with a drum crown for exact offsetting of the degree of thermal expansion, for example in the case of austenitic stainless steel, as shown in FIG. 4, a phenomenon occurs wherein the thickness of the portion of the thin cast strip 6 from the edge to 50 mm in the widthwise direction becomes enlarged. In the case of excessive enlargement, another phenomenon has occurred in which the edges of the thin cast strip drip off directly under the cooling drum. The enlargement will hereunder be referred to as "edging up", and dripping off of the edges will be referred to as "edge loss". The difference between the maximum thickness A of the edged-up sections and the thickness B of the edges of the thin cast strip with no influence by edging up (A-B) will be defined as the "edging up height".
When edging up and edge loss occur, it becomes difficult or impossible to roll up the cast strip. Inadequacies in the shape of the final product plate, naturally, will often make it impossible to accomplish roll forming by final rolling. This also can become a cause of cracks and wrinkles in the thin cast strip surface. Much trimming and surface grinding is necessary to avoid these problems, and this both complicates the process and lowers the yield.
It is, therefore, an object of the present invention to obtain a thin cast strip with a satisfactory shape while preventing edging up and edge loss of a thin cast strip formed of molten steel when thin cast strip is produced with a twin drum-type continuous casting apparatus.
It is another object of the present invention to prevent occurrence of cracks and wrinkles in the thin cast strip to provide products with satisfactory surface quality.